1. Field of the Invention
The present invention relates to a control device for an internal combustion engine which is provided with a variable valve drive mechanism, and more specifically, to an estimation device for a cylinder intake air amount in an internal combustion engine, which serves for calculating an amount of intake air in a cylinder with a high degree of accuracy.
2. Description of the Related Art
In general, in order to control an engine in a suitable manner, it is important to calculate an amount of air to be sucked into a cylinder with a high degree of accuracy, and to carry out fuel control and ignition control according to the amount of air which has been sucked into the cylinder.
In the past, there has been known a technique (an AFS method) in which in order to measure an amount of intake air sucked into a cylinder of an internal combustion engine, the amount of intake air is measured by using an air flow sensor (hereinafter referred to as an “AFS”) which is arranged in an intake pipe of the engine at a location upstream of a throttle valve.
In addition, there has also been known another technique (an S/D method: Speed Density method) in which a pressure sensor (hereinafter referred to as an intake manifold pressure sensor) is provided for measuring the pressure in a portion (which is generically referred to as an “intake manifold part”) including a surge tank and an intake manifold, which are arranged at the downstream side of a throttle valve, so that an amount of intake air sucked into a cylinder is estimated from the intake manifold pressure thus measured by the intake manifold pressure sensor and an engine rotational speed which is measured separately.
Further, there have also been known a technique which serves to switch between an AFS method and an S/D method according to an operating state of an internal combustion engine by using the above-mentioned two sensors in combination with each other, and a technique which serves to measure an intake manifold pressure, even in the case of an AFS method.
In recent years, in order to further reduce fuel consumption as well as to further increase output power, there becomes popular an engine which is provided with an intake VVT as a variable valve timing mechanism (Variable Valve Timing) (hereinafter referred to as a “VVT”) which serves to make variable the valve opening and closing timing of each intake valve. Moreover, an intake and exhaust VVT system is becoming increasingly adopted in which in addition to an intake valve, an exhaust valve is also provided with a VVT mechanism (i.e., an exhaust VVT).
In an engine provided with such an intake and exhaust VVT system, an amount of intake air sucked into a cylinder from an intake manifold changes to a large extent depending on the valve opening and closing timing of the intake and exhaust valves, as a result of which if the influence of the valve opening and closing timing is not taken into consideration, in the AFS method, the calculation accuracy of the amount of intake air sucked into the cylinder will decrease in a transient operation region such as at the time of acceleration and deceleration, etc., whereas in the S/D method, the calculation accuracy of the amount of intake air sucked into the cylinder will decrease in all the operation regions including a steady-state operation region and the transient operation region.
Accordingly, in an engine provided with such an intake and exhaust VVT system, too, there has been proposed a technique in which in order to calculate the amount of intake air sucked into a cylinder with a high degree of accuracy, a physical model representing the behavior of air in an intake and exhaust system such as an intake passage, an intake valve, an exhaust valve, etc., is constructed by the use of expressions which are obtained based on physical laws such as the law of conservation of mass, the law of conservation of energy, the law of conservation of momentum, etc., in the AFS method, and the amount of intake air sucked into the cylinder is estimated by using this physical model (for example, refer to a first patent document).
However, in the technique described in the first patent document, the amount of air sucked or drawn into the cylinder while passing through the intake valve and the exhaust valve is calculated by calculating the expressions (refer to paragraphs [0072]-[0188] in the first patent document) obtained based on the physical laws such as not only the law of conservation of mass but also the law of conservation of energy, the law of conservation of momentum, etc., at each predetermined time interval. As a result, there is a problem that it becomes necessary to carry out a huge amount of arithmetic calculation operations.
In particular, in recent years, improvement in the operation speed of microcomputers used in engine control units (hereinafter referred to as ECUs) is being carried out, but at the same time, contents of various kinds of processing other than the estimation processing of the amount of intake air also become complicated, so there is a problem that it is difficult to spend a huge amount of arithmetic operation or calculation load only on the estimation of the amount of intake air.
In addition, the model described in the first patent document has not only physical properties such as specific heat at constant pressure, specific heat at constant volume, etc., but also many coefficients or factors such as a flow rate coefficient, etc., for which adaptation is required, and besides, in order to adapt them to actual values, it is necessary to calculate instantaneous pressure and instantaneous temperature for which it is difficult to obtain strict measurement values with high accuracy. As a result, there is a problem that the amount of man hours required for adaptation also becomes huge, and besides, the verification of the physical model itself is difficult.
Accordingly, as a simplified physical model, there has also been proposed a technique in which in the AFS method, an intake system is modeled only from the law of conservation of mass (for example, refer to a second patent document).
In the technique described in the second patent document, by using a volumetric efficiency corresponding value (a volumetric efficiency correction factor Kv) of the air which comes into a cylinder from an intake manifold, an amount of intake air sucked into the cylinder is estimated by means of a simpler physical model with a sufficient degree of accuracy for controlling an engine in a suitable manner, without carrying out complicated arithmetic calculation operations such as those for the law of conservation of energy, the law of conservation of momentum, etc. (refer to paragraphs [0023], [0024], and [0038]-[0042] in the second patent document).
Here, note that the volumetric efficiency correction factor Kv is stored as one map in which two orthogonal axes are represented by an engine rotational speed and an intake manifold pressure, respectively, in an engine which is not provided with an intake and exhaust VVT system.
Moreover, in the second patent document, there is also described a construction example (refer to paragraph [0067]-[0071]) in which an application is made to an engine provided with a VVT mechanism, but in this construction example, it is necessary to store a volumetric efficiency correction factor map as a map for every operating state of a variable valve.
Specifically, in cases where the operating range of the variable valve is represented by six representative points, and is used with each region between adjacent points (hereinafter referred to as an interpoint region) being interpolated, for example, six volumetric efficiency correction factor maps are required for a system configuration using only an intake VVT, and a total number (6×6=36) of volumetric efficiency correction factor maps will be required for a system configuration of an intake and exhaust VVT system.
Accordingly, in the case of the above-mentioned configuration examples described in the second patent document, the system configuration using only the intake VVT is considered to be within the range of practical use, but in the system configuration of the intake and exhaust VVT system, the number of volumetric efficiency correction factor maps becomes huge, thus giving rise to a problem that a large number of man hours are required in adaptation or data setting, and at the same time, the memory capacity necessary for a microcomputer of an ECU also becomes huge.
On the other hand, a simplified physical model in the S/D method has also been proposed (for example, refer to a third patent document).
In this third patent document, there is shown that an amount of intake air sucked into a cylinder is calculated from an intake manifold pressure MAP, a volumetric efficiency VE, a cylinder volume V and a temperature T (refer to paragraphs [0003], [0004], and [Expression 1] in the third patent document).
Here, note that expression 1 described in the third patent document and expression 2 described in the second patent document mean the same thing or content, when the relation of the equation of state of a so-called ideal gas (P=ρRT, where P: pressure, ρ: density, R: gas constant, and T: temperature) is taken into consideration, and the volumetric efficiency VE described in the third patent document is considered to be the same thing as the volumetric efficiency correction factor Kv described in the second patent document.
Accordingly, in the third patent document, it is assumed as a premise that engine parameters such as engine valve timing, etc., do not change, but if a variable valve is applied to the S/D method of the third patent document, for example, the number of maps for the volumetric efficiency VE will eventually become huge, as in the case of the second patent document, thus giving rise to a problem that a large number of man hours are required in adaptation or data setting, and at the same time, the memory capacity necessary for a microcomputer of an ECU also becomes huge.
That is, in an internal combustion engine which is provided with a variable valve drive mechanism, in cases where an amount of air actually sucked into a cylinder is estimated by a physical model of an intake system using a volumetric efficiency corresponding value which is an index indicating an amount of air coming into the cylinder from an intake manifold, the volumetric efficiency corresponding value changes according to the actual valve timing of the variable valve drive mechanism. As a result, in order to calculate the volumetric efficiency corresponding value with a high degree of accuracy, it is necessary to adapt the volumetric efficiency corresponding value according to the valve timing, and hence, the number of maps for storing these values becomes huge.